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Title of the seminar 8 size

ACKNOWLEDGEMENT

With great pleasure I want to take this opportunity to express our heartfelt

gratitude to all the people who helped in making this seminar work a grand success.

I express our deep sense of gratitude to Prof. B. Karunaiah

for his constant guidance throughout our seminar work.

I would like to thankProf. K. V. Murali Mohan, Head of the Department

of Electronics and Communication Engineering, for being moral support

throughout the period of our study in HITSCOE.

First of all I am highly indebted to PrincipalDr. N. Subhash Chandra, for

giving me the permission to carry out this seminar.

I would like to thank the Teaching & Non-Teaching staff of ECE

Department for sharing their knowledge with me.

Last but not the least I express our sincere thanks to Mr. A. Vara Prasada

Reddy Chairman and Mrs. A. Vijaya Sarada Reddy Secretary, HITS Group of

Institutions, for their continuous care towards my achievements.

Name of the student

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CONTENTS

Abstract 2

1. Introduction - Typical Wafer Level Camera Design

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2. CMOS Sensor Packaging (Glass encapsulation)

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2.1 Fabrication of optical components via UV

replication 4

2.2 Wafer Level Packaging of Opto-Wafers

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3. Fast UV-curable materials for Wafer Level Camera

Manufacturing 8

3.1 Characterization of Micro Lenses

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4. Conclusion 14

5. References 14

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Abstract

The increasing demand for more functions and featurescoming along with cost reduction plays a significant role in todaysproduct design and manufacturing technologies of mobile devicessuch as PDAs, laptop computer and mobile phones. Besides themain function of the device, imaging is considered as core featureby the user and major mobile phone manufacturers. Therefore theindustry puts a lot of effort onto performance improvements and theoptimization of the manufacturing method of mobile phonecameras. Wafer Level Camera (WLC) is supposed to be the

technology of choice to address these requirements.

In recent years Wafer Level Packaging of CMOS image sensorshas become a well established technology in the industry. Thistechnology provides a cost efficient packaging method for shrinkingdevices sizes coupled with higher I/O density.

In addition to this Wafer-level optics is a novel technology thatis designed to meet the demand for smaller form factors of theoptical system and cost reduction in the next generation of cameraphones. The optical components are fabricated by replicating the

optics through a stamp material into a polymer layer, coated on aglass wafer. Another key challenge is the wafer alignment.Replicated lens wafers are aligned and adhesively bonded at thewafer level using a UV curing process in order to achieve excellentalignment results. Finally the bonded Opto Wafers are subsequentlydiced to form individual camera modules.

This seminar report explores the latest fabrication techniquesas used in the Wafer Level Cameras (WLC) where Opto Wafers andCMOS-Wafers are mounted by Wafer Level Packaging (WLP) and

describes all the challenges and available solutions. The processingissues encountered in those techniques are discussed with a focus

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on each WLC process step. A typical Wafer Level Camera layout isdescribed; the replication of microlenses and the packaging of suchmicrolens wafers (Opto Wafers) via UV curing is depicted as well.

Also wafer level packaging of the CMOS wafer using bonding

techniques is part of this paper. UV curable materials for microlensreplication and for Wafer Level Packaging of Opto Wafers (lensstacking) is presented as well. Optical measurement technology forquality assurance of micro-lenses finally concludes the report.

1. Introduction - Typical Wafer Level CameraDesign

Typically a wafer level camera consists of two main pieces, theimage sensor and the optics. Figure 1 shows such a schematic crosssection of a classical wafer level camera.

Figure 1. Schematic of Typical Wafer Level Camera Design

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Figure 2. 8 Lens Master (left) and replicated 8 Wafer (right) [2]

2. CMOS Sensor Packaging (Glass encapsulation)

In a camera device the CMOS image sensor needs to becovered by a glass layer to protect the active area. This is typicallydone on Wafer Level using bonding techniques. The challenges inthis process are high alignment accuracy and excellent temperatureand pressure uniformity to achieve best yield. Void-free bondinterfaces are also required. The most popular process is adhesivebonding, chosen because of low bonding temperature (below200C). Typically the adhesive is dispensed or rolled on frameslocated at one of the wafers.

The next step is alignment of both wafers. As one of thewafers is transparent (glass) a live alignment is feasible. Due to thesensor dimensions typically the required post bond alignmentaccuracy is below 10m. Followed that thermal curing takes place inthe bonding chamber, SUSS SB8e. The given pressure uniformity of1,5% leads to excellent bonding results. In return the optics ismanufactured using other machine types, SUSS MA8 Gen3Maskaligner equipped with dedicated tooling.

2.1 Fabrication of optical components via UV replication

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The replication of optical parts like lenses for WLC is mostlydone by molding them from a stamp. Dependent on the method tobe used while replicating the lenses, two different stamps are used.

Hard stamps mainly produced from glass, when the process

involves squeezing a dispensed polymer droplet over the wafer areaor the structures are embossed into a polymer layer spread over thewafer surface in advance as it is frequently found in hot embossing.Soft stamps, usually produced from silicone rubbers (i.e. PDMS),when the polymer is dispensed into the single lens molds and thelenses are casted by transfer of these polymer droplets onto thesupporting wafer (depicted in figure 3). Both methods have theirpros and cons.

Figure 3: Comparison between the main imprinting methods.Embossing with residual layer on the left, transfer print without

residual layer on the right

The glass stamp provides a very good control about its totalthickness variation (important for the uniformity of the moldingresult and for the alignment accuracy, as described below) andenables the replication of even the smallest structures due to its

high stiffness and resulting contour accuracy. On the other hand thehard stamp requires a base layer as a conformal printing over thewhole wafer area is impossible between two hard surfaces.Additionally, imprinting into a predispensed layer requires big forcesto displace the material. Machines supporting this imprint methodhave to be very warping resistant and need actuators that canprovide these high forces and allow an active control of wedgeerrors.

The silicone rubber stamp shows drawbacks as the hard tocontrol total thickness variation, the shrinkage that can appearduring stamp production and the worse resolution due to possibledeformations in the stamp during imprinting. However, due to its

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soft surface that adapts to the wafer surface it can produce lenseswithout the presence of a residual layer. Therefore wafer warpagedue to shrinkage of the lens material is strongly reduced. As a sideeffect also particle contamination is a less serious problem with softstamps, leading to reduced clean room costs.

As mentioned above, the total thickness variation is not only aproblem for the uniformity of the imprint result, but can also causeproblems as soon as the replication process includes alignmentbetween the lens layer and the wafer.

While a wedge in the stamp can still be compensated by thewedge error correction system of modern mask aligners, substratewarpage or deformations of even higher polynomial grade cannot becompensated by the machines. Therefore contact between differentpoints on the wafer area is established at different times and leadsto lateral forces between wafer and stamp which are hard to control.These lateral forces are mainly caused by shearing of the stampmaterial due to the non-uniform load or by viscous forces caused bythe non-isotropic flow of glue between the stamp and the wafer.

Due to these forces shifts between stamp and wafer can occurduring the imprint process. To compensate for the shifts, processeswith multiple approaching and realigning steps are necessary.Especially during the last steps of such an imprinting process thecontrast of the fiducials, which are typically produced in polymer

during the imprinting process themselves, can be very week. Hereonly the combination of well chosen microscopic techniques andmade to purpose fiducial design can provide images offeringsufficient reliability for automated alignment. Fiducial geometryshould take care about reducing deformations of the polymerpattern as well as about the needs of the pattern recognitionsystems.

To achieve highest reliability, the pattern recognition systemsneed expanded structures. Those expanded polymer fiducials maybe created by dispensing opaque polymers onto the fiducial region

or by creating rough surfaces which cause increased scattering, butfurther development is needed at this point (figure 4).

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Figure 4: schematic drawing of polymeric alignment marks andtheir microscope images. a) mark as designed, b) left half: veryweak contrast as obtained with standard reflected light illumination,right half: stronger contrast with transmitted light. Still the patternis not ideally suited for pattern recognition due to its narrowcontrasted structures. c) internal surface patterning of the target toenhance contrast and create expanded structures d) contrast as

obtained with DIC microscopy or similar method resulting in bettercontrasts and expanded structures

Also transmitted light microscopy can help to increasestructure contrast. Besides that, the use of microscopy optics withsmall numerical apertures and advanced illumination techniques asdark field or DIC (Differential Interference Contrast) may be usefuland are therefore options in SUSS machines.

2.2 Wafer Level Packaging of Opto-Wafers

Certainly camera systems consist of several optical elementsthat need to be assembled quite accurately to provide the bestpossible optical performance. Until today camera systems havebeen manufactured by manually assembling lenses into a barrel.This procedure is very costly, time consuming and doesnt seem tobe a reasonable approach for the manufacturing of multi level,miniaturized lens stacks that are supposed to be use in moderncamera systems for mobile phones.

Wafer level bonding technologies seem to solve this issue. The

industry started to use wafer bonding equipment platforms to bondlens wafers with spacer wafers or a second lens wafer with thermalcurable adhesives. For this technology state of the art bond aligningand bonding tools can be use. The wafers with applied adhesive onone of the wafers get aligned, get clamped in a transport fixture andget finally bonded in a substrate bonder. However, this process haslimits in terms of the achievable alignment accuracy. The reason isthat thermal stress in the bond process and the required handlingsfrom the bond align to the bond tool impacts the alignment accuracyand limits process reliability.

As lens wafers are transparent for UV light the usage of UVcurable adhesives seems to be the solution of choice for a cost

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effective and highly accurate wafer level assembly with in-situalignment on a mask aligner type of equipment. Leading edge maskaligner technologies allow alignment accuracies well below 0.5mand offer high intensity UV illumination for effective UV curingprocesses. The sequence of a UV bond is very similar to common

mask aligner photolithography. Two substrates have to be aligned inan accurately controlled alignment gap and UV exposure finalizesthe process step. However, for UV bonding the Mask Aligner requiresa specific substrate holder that includes a UV transparent chuckingplate to hold the top lens wafer. This is shown in figure 5. The lowerlens wafer is handled onto an exposure chuck which is typicallydesigned for wafer edge handling or for the usage of buffer wafer.Buffer wafers are typical spacer wafers (glass wafers with holes atthe position and with the size of the lenses) that are needed andused to ensure a well defined and controlled distance between lenswafers. Both are dedicated to safely handle the wafers withreplicated convex lenses.

Figure 5: UV bonding incl. in-situ alignment on the SUSS MA8 Gen3with the usage of buffer wafers

Besides the alignment and bond equipment, the dispensetechnology and process play a significant role in the manufacturingof WLC. The dispense volume and pattern of the adhesive need tobe controlled to achieve a void free and reliable bond interface. Toomuch material results in contaminated optical elements while tooless material results in leakages of the module itself. The adhesiveitself needs to be chosen carefully to fit to the general requirements

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to be a fast curing, highly reliable, dispensable and last but not leastreflow compatible material. In addition material suppliers like DELOoffer material with integrated filler particles to achieve a uniformand automatic residual thickness control by the material itself.

With the use of Opto wafers with lenses that are embedded inthe polymer, roller dispense processes can be adopted. This offer amuch easier dispenses process but limits the design of the lenswafer.

High accuracy alignment of Opto wafers can be achieved onmanual and automatic mask aligners from SUSS MicroTec. Asdescribed above, sophisticated toolsets are used to safely handlethe lens wafers and to ensure an excellent leveling of both wafersduring the gap setting and alignment step. One of the keychallenges at the alignment process is the very large distancebetween alignment fiducials. Depending on the number of wafers tostack, the distance between the alignment targets can get up toseveral mm. The optical alignment system incl. the microscope andfocus settings need to be highly accurate in design and setup. Inaddition, leading edge technologies like SUSS Assisted Alignment,which provides live overlay measurements and direct operatorfeedback, are required to achieve alignment accuracies


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Figure 6:Alignment results obtained during UV-stacking on a SUSSMA8 Gen3. The points represent results measured with the cognexsystem in the machine. Vernier measurements resulted in alignmentaccuracy better than the 0.4 m resolution of the Vernier (red area)

3. Fast UV-curable materials for Wafer LevelCamera Manufacturing

UV curing adhesives are currently widely used for massproduction in the electronics and optics assembly industries.Depending on the chemical basis acrylic or epoxy- these materialsdiffer in some of the basic parameters.

Acrylic adhesives are known to be very fast curing, but arelimited at high temperature processes and have high polymerization

shrinkage. On the other hand, epoxy based system are known forgood thermal stability and low shrinkage. Therefore they are used inhigh reliability applications, like automotive and optical assembly.

The challenge for Wafer Level Optics (WLO) manufacturing isto develop materials with fast curing mechanism, high opticaltransmission and high thermal stability (reflowable optics).

The following results show the current status of the materialdevelopment within DELO:

a. Fast curing with good adhesion

Due to the reflow requirements in the WLC module and thehigh throughput needed for low cost wafer level manufacturing,DELO has developed new fast UV-curing epoxy-based adhesives.Referring to figure 7 initial strength of Glass/ Glass samples isreached very fast, at 10 sec after exposure.

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Figure 7. Time of reaching initial Shear Strength after UV exposure,compared to standard UV curing epoxy

b. Adhesion to Stamp MaterialFor imprint materials, it is very important to have excellent

adhesion to glass (wafer), whereby the adhesion to the stampmaterial should be as low as possible. In our tests, all printed opticswafers could be easily removed from the used stamp material (2

component - silicone).

c. Low Outgasing and Low ShrinkageAnother important parameter in optics applications is the

outgasing of the adhesive at high temperatures. Looking at processtemperatures up to 260C, one can see, that the weight loss of theDELO KATIOBOND AD VE 18499 is


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Figure 8. TGA Curve of DELO KATIOBOND AD VE 18499

The shrinkage was measured at 3 positions on a wafer during theimprint process. Thickness shrinkage was < 1.5% (Table 1).

Table 1. Thickness change at Curing of DELO KATIOBOND AD VE18499

d. Reliability Testing

- Optical stabilityTransmission Measurements were done on 100m thick

foils of cured adhesive with no protection. The foils wereanalyzed after:

a. 168h Xenon Solar Light exposureb. 2min @ 270C (Reflow Simulation)c. 168 h 85C/ 85% r.H.d. 168h 125

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As can be seen on Figure 9, an optical transmission of largerthan 80% over the visible range (400nm 750nm) was achieved forall the conditions tested.

Figure 9. Transmission of 100m thick foils of DELO KATIOBONDAD VE 18499

- Thermal stabilityIn order to simulate the mechanical stress in a wafer

level optics module, 2 different samples (A, B) were tested:A: 20mm x 20mm x 5mm glass plates with a 500madhesive layerB: 4mm x 4mm x 4mm glass cubes with a 100madhesive layerStress on both samples was introduced by a grinding

process to simulate the sawing process and to generatepossible micro cracks at the edges of the sample.

Samples A were tested after 500h 85C/ 85% r.H.The optimized DELO adhesive did not show any delamination, incontrast to some of the standard adhesives.

Samples B were run through the following conditions:

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- Temperature shock test: -40C 85C

The left picture in figure 10 shows delamination of thestandard adhesive at the edge of the sample. The right picture infigure 10 shows optimized adhesive, where no delamination occurs.

Figure 10. Images after 300 temperature shock cycles [m]- Reflow Condition

A typical reflow profile was simulated in a temperaturecontrolled chamber. None of the samples did show

delamination.

3.1 Characterization of Micro Lenses (Master, SiliconStamp and Replicated Lens)

For the measurement of surface deviations the Twyman-Green Interferometer is best suited (Prof. J. Schwider, UniversityErlangen). The light of a partially coherent source is used toilluminate the interferometer. The deviation of the reflected beam inthe test arm (condenser objective / spherical surface) from a planewave provides the information on the deviations from the sphericityof the micro surface.

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Figure 1: Twyman-Green Interferometer for measuring surfacedeviations and radius of curvature.

Besides of the measurement of surface deviations the radiusof curvature of a micro sphere can be determined with the help ofthis interferometer (= difference between basic position and catseye position). The just described interferometer was used tocompare the shape of master, stamp and replicated micro lenses.For the fabrication of the stamp and the replicated lenses the newMask Aligner MA/BA8 Gen3 from SUSS MicroTec was used. Thereplicated lenses were fabricated in UV curing adhesives from DELO.The following table shows measurement values (radius of curvature,deviation from ideal sphere) over 5 points of a 4 inch wafer (centre,right, bottom, left and top) and gives an idea about the very gooduniformity which was achieved.

Table2: Radius of curvature and surface deviation from ideal

sphere ( = 633nm) for 5 points measured for master, stamp andreplicated wafer.

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4. Conclusion

In this paper novel fabrication and packaging technologieswere introduced. Some required equipment characteristics wereintroduced, which are beneficial for processes in creating wafer levelcameras, like the in-situ alignment and UV-bonding in one tool.

UV curable materials for microlens replication and for WaferLevel Packaging of Opto Wafers were presented with their attainableparameters. The developed new materials are showing very good

optical and mechanical reliability, as well as excellent reflowbehavior. Therefore they were selected for further testing for theimprint processes.

Finally characterization of micro lenses was discussed andmeasurement values of master, stamp and replicated wafer wereshown.

5. References

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[1]Reinhard Voelkel, Ralph Zoberbier; "Inside Wafer- LevelCameras", Semiconductor International, February 2009, pp. 28-32,2009[2] Niels-Christian Rmer Holme, Palle Geltzer Dinesen, Ziv Attar,Steven D. Oliver, Reinhard Voelkel, "New technologies enable

precise and cost effective waferlevel optics", Laser Focus World,January 2009

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